Baroness Smith, Labour’s leader in the Lords and a former Northern Ireland minister, told HuffPost: “This is shocking. Such offensive comments could never be considered appropriate - on Twitter or anywhere else.”

Former Northern Ireland Secretary Peter Hain attacked Kilclooney’s remark as “racist” and said it underlined how thin were the arguments of those criticising the Irish government’s stance on Brexit and a hard border.

One Cabinet minister also told HuffPost that the tweet was “truly appalling and unacceptable”.

Downing Street said that Theresa May had not yet seen the remark, but stressed “the importance of anyone engaged in public debate being careful in the words they choose”.

Lord Kilclooney, formerly John Taylor MP, told BBC News NI he rejected “false accusations of racism”. He also said that he did not intend to withdraw the tweet.

All of that confirms he is a 100 per cent Irish Citizen but he has said himself that he is half Indian and in my opinion only full Irish have an understanding of politics in this island - not even the English understand!

I am certainly no racist and in particular have an admiration for Indians.a member of the British/Indian APPG only yesterday I had a reply from 10 Downing St asking for a relaxation of visas for Indians. My point was that the PM had upset Unionists more than Irish PMs had!

He also stressed that he felt that Varadkar had been ‘provocative’ in staging his trip to Northern Ireland without consulting local politicians, even though the Northern Ireland Office had been informed.

Alliance Party leader Naomi Long described the tweet as a “racist slur”, adding it was “insulting of not only the Taoiseach and the Irish people, but of Indian people too.”

Lord Kilclooney's tweet is a racial slur, insulting of not only the Taoiseach and the Irish people, but of Indian people, too. There is no explaining this one away like last time.

The HoL needs to address his conduct: it is deliberate, calculated disrespect.

Cross-party condemnation of Kilclooney remarks in the #Seanad this afternoon. I’ve written to An Cathaoirleach asking that he convey our shared disgust to his counterpart in the British House of Lords. @sinnfeinireland

Lord Hain, who served as Secretary of State under Tony Blair, told HuffPost: “[This was] a racist jibe, springing from an unwillingness by too many Unionists to acknowledge that Varadkar is correct when he says that the only way to keep the Irish border open is to replicate the customs, trade, regulatory and rules of origin arrangements that keep it open now.”

He later withdrew the tweet, and a complaint to the House of Lords standards commissioner was dismissed.

At the time, Varadkar hit back, saying: “One thing I have come to understand about politics is that when people don’t want to deal with you on substance and on facts they attack you personally and attack your style.

“The best way to deal with it is not rise to it and go back to the substance and to ask them 18 months after the referendum ‘show us your paper or your proposals’ - 20 years since you started to campaign to leave the EU.

“And that makes them very annoyed and they start having a go at you personally - so many of them haven’t thought this through.”

A spokeswoman for the Lords Commissioner for Parliamentary Standards pointed out that its code of conduct made clear that “a member’s views or opinions” were outside its remit.

There is also no catch-all rule barring conduct that brings the Lords into disrepute.